Haven't seen Australia but from what I've heard, I think I might like it. I'm not hopeful for The Great Gatsby but maybe I'll be surprised.
If you can make it past the first 20 or so minutes, Australia is a pretty good movie. However the first 20 minutes is so tedious, and over rough you will want to give up on it. In fact if you did, I wouldn't blame you.
I'm very hopeful for The Great Gatsby. From what I have seen, while looking like classic Baz Luhrmann it does look toned down.